Add tests for negative thresholds in test_utils
Add tests for negative thresholds in check_snmp

Next commit will work on fixing these!
Handle negative values properly with check_snmp

check_snmp becomes capable of evaluating negative values properly,
but it might be returning CRITICALs where it used to return OK and was ignored,
if a negative value turns out to actually be a valid value.
If negative values are valid, this can be worked around,
by adding "~:" to the warning/critical threshold : 100 -> ~:100